Filtration is an essential step in treating drinking water and industrial water. Filtration is carried out to remove contaminants that may be introduced with source water and/or may be generated during a water treatment process. Large-scale filtration commonly involves passing water through granular filter media, such as various types of sand, anthracite coal, granular activated carbon or combinations thereof arranged in one or more layers within a filter media bed, or though media such as ion exchange resins and/or ceramic beads.
Contaminants removed during filtration accumulate within a granular filter media bed. Over time, this accumulation leads to increased filter backpressure (measured by increased head loss), increased turbidity of flow-through, or (in a worst case scenario) breakthrough of contaminants. Regular backwashing is commonly used to remove accumulated filtrate in an effort to maintain filter performance and capacity.
Filter media particles typically accumulate deposits of biological and non-biological material that cannot be removed by backwashing, and that can significantly interfere with the filter's function. To maintain acceptable filter performance and capacity, it is important to periodically remove surface deposits that are not removable by backwashing.
Depending on the water source and environmental conditions, surface deposits on the filter particles can consist primarily of organic matter (biofilm), metal oxides, and/or calcium carbonate scale. Surface deposits on filter particles can also be formed due to pretreatment steps, such as may utilize aluminum-based pre-oxidant compositions. Heavy fouling or scaling will eventually reduce filter performance, with consequences including higher backwash frequency, reduced flowrate, increased water turbidity, breakthrough of contaminants and/or a combination of the foregoing. If filter deposits are not removed, then filter performance will eventually decline below tolerable levels and filter media exchange becomes necessary.
Unfortunately, exchange of filtration media is very expensive—including not only labor costs and direct costs associated with purchasing filtration media, but also substantial indirect costs associated with filter downtime. Exchange of filtration media may not be a practical maintenance solution for a water treatment installation having a single filtration bed and that serves as an exclusive water source for particular consumers. As a result, there is a need for maintenance procedures that allow for the cleaning of surface deposits from filtration media, as an alternative to filter media replacement.
Traditional mechanical methods for filtration media cleaning have included aeration of filter beds during backwash, and spraying water on top of filter beds to disperse soft aggregates. Such methods are not suitable for removing persistent surface deposits such as biofilm and scale.
Traditional chemical treatments for filtration media having included washing filtration media strong acids and bases, sometimes in combination with surfactants. These chemical treatments can be satisfactory for certain types of contamination, such as calcium carbonate scale; however, mixed deposits, which include metal oxides and biological films, are either not removed efficiently or require highly corrosive and hazardous cleaning agents that are difficult to use and may leave residue not acceptable in drinking water processing installations. In certain instances aqueous solutions of strong acid or strong base have been applied to filtration media; however, the treatment liquid may flow too quickly through the filtration media for a cleaning reaction to be completed. To obtain desired cleaning performance, treatment liquid could be applied repeatedly (thus requiring vast amounts of treatment liquid), or treatment liquid could be circulated through the filtration media until the clearing reaction is completed (requiring specialized and expensive circulation equipment). These approaches, although presenting a potential alternative to extended plant shutdown for filter media replacement, do not offer an economical alternative to filter media replacement.
Additional filtration media cleaning methods and compositions are disclosed in U.S. Patent Application Publication No. 2008/0006589A1 to Reimann-Philipp, et al., entitled “Process for In-Situ Cleaning of Drinking Water Filtration Media.” Such publication discloses application of a granular cleaner to water filtration media and then applying a granular or liquid activator embodying an activated oxygen donor to the water filtration media (with the granular cleaner preferably being wetted after application) to cause a chemical reaction between the granular cleaner, activator, and water filtration media, resulting in cleaning of the water filtration media. Preferred constituents of the granular cleaner include sulfamic acid (50-99 wt %), citric acid (0-10 wt %), phosphoric acid (0-10 wt %), corrosion inhibitor (0-10 wt %), free-flow additive (0-10 wt %), surfactant (0-10 wt %), and sodium bicarbonate (balance). Particularly preferred constituents of the activator include either 5-50% hydrogen peroxide, or 0.2-10% peracetic acid, or a combination of hydrogen peroxide and peracetic acid, with the balance being water. If the activator is in granular form, then the activator and cleaner may be mixed prior to application, but in a most preferred embodiment, the activator is applied as an aqueous solution to a granular cleaner present in a filtration bed. The granular cleaner can also be applied as a slurry.
Although the compositions and methods disclosed by Reimann-Philipp, et al. provide improved cleaning performance relative to the traditional mechanical and chemical methods outlined above, such compositions and methods include certain drawbacks. First, it can be difficult to ensure that correct proportions of cleaner and activator are consistently used. Second, it can be cumbersome and/or difficult to apply separate cleaner and activator components—and particular difficult to do so with uniformity. Third, the granular cleaner may be only partly soluble in water, thereby resulting in application problems. Fourth, it can be difficult to ensure the entire volume of filtration media with a filtration bed is cleaned, particularly if cleaner and activator may react rapidly with one another before penetrating an entirety of a filtration bed. Fifth, it can be cumbersome to monitor progress of a filtration bed cleaning process. Sixth, it can be costly or cumbersome to transport large volumes of liquid activator. Seventh, conventional cleaning methods utilizing hazardous chemicals requires personnel to undergo significant training and undertake hazard protection steps. Eighth, it can be challenging to avoid formation of foam and/or other waste products.
Given the foregoing, there remains a need for alternative cleaning compositions and methods with enhanced efficiency and/or convenience to present an alternative to media replacement and an alternative to existing in situ filter cleaning compositions and methods. It would also be desirable for cleaning methods to avoid or reduce formation of byproduct deposits that may be more durable than deposits initially sought to be removed by a filter media cleaning process.